It seems like you got it figured out, you just needed an example of how to write it.
As you noticed, the "on change do" event handler inside the slider will usually only trigger when it's clicked/interacted with directly.
So here's an example of how to cause the "change" event to happen from inside a button script:
on click do slider1.value: slider1.value-1 slider1.event["change" slider1.value] end
Hopefully this gives you what you needed. Looks cool!